The present invention generally relates to high electron mobility transistors, and more particularly to a high electron mobility transistor which uses as a channel a two dimensional electron gas layer formed in a vicinity of a hetero interface.
Conventionally, a high electron mobility transistor (hereinafter simply referred to as a HEMT) is known as a high-speed field effect transistor. The HEMT generally comprises a semiinsulating gallium arsenide (GaAs) substrate, an undoped (or intrinsic) GaAs channel layer formed on the semiinsulating GaAs substrate, and an n-type aluminum gallium arsenide (AlGaAs) carrier supplying layer. A two dimensional electron gas layer formed in a vicinity of a hetero interface on the side of the undoped GaAs channel layer is used as the channel between a source and a drain. The density of two dimensional carrier gas in the two dimensional electron gas layer is controlled by a voltage applied to a gate which is formed on the n-type AlGaAs carrier supplying layer. In addition to the combination of GaAs having a large electron affinity and AlGaAs having a smaller electron affinity, the combination may be GaAs and indium gallium phosphide (InGaP), indium gallium phosphide (InGaAs) and indium phosphide (InP), or indium gallium arsenide (InGaAs) and indium aluminum arsenide (InAlAs).
According to the conventional HEMT, the energy difference between the conduction bands of GaAs and Al.sub.x Ga.sub.1-x As(x=0.3) is approximately 0.2 eV and is extremely small. Even in the case of the combination of GaAs and InGaP, the energy difference between the conduction bands is approximately 0.4 eV and is not much larger.
Generally, when the energy difference between the conduction bands is small, a known phenomenon occurs wherein the carriers accelerated in the two dimensional electron gas layer are transferred to the carrier supplying layer and the carrier mobility becomes low. This phenomenon is referred to as the real space carrier transfer to carrier supplying layer. Therefore, when the energy difference between the conduction bands is large, the carriers in the two dimensional electron gas layer are prevented from moving into the carrier supplying layer even when there is a high voltage applied across the drain and source. For this reason, the HEMT can operate stably even when a high voltage is applied across the drain and source. On the other hand, the density of the two dimensional carrier gas in the two dimensional electron gas layer of the HEMT is proportional to the energy difference between the conduction bands. Consequently, the larger the energy difference is between the conduction bands, it is possible to obtain a larger current output.
The Japanese Laid-Open Patent Application No. 58-140167 discloses a HEMT wherein undoped InGaAs, indium gallium arsenide phosphide (InGaAsP) or the like is used as the channel layer and doped InGaAsP, zinc selenide (ZnSe) or the like is used as the carrier supplying layer. According to this previously proposed HEMT, it is possible to obtain a large energy difference between the conduction bands compared to that obtainable in the conventional HEMT described before. However, because the channel layer is a three-element or four-element layer, there are problems in that scattering of alloy occurs at a low operation temperature, and the carrier mobility becomes low. For example, when undoped InGaAs is used as the channel layer and doped InAlAs is used as the carrier supplying layer, it is only possible to obtain a carrier mobility .mu. in the order of 5.times.10.sup.4 cm.sup.2 /Vsec to 6.times.10.sup.4 cm.sup.2 /Vsec at a temperature of 77.degree. K. On the other hand, the carrier mobility .mu. obtainable in the conventional MEHT using undoped GaAs as the channel layer and doped AlGaAs as the carrier supplying layer is in the order of 10.sup.5 cm.sup.2 /Vsec to 2.times.10.sup.15 cm.sup.2 /Vsec at the temperature of 77.degree. K.
That is, in the case where the channel layer is made of a three-element compound semiconductor, the carrier mobility is low compared to the case where the channel layer is made of a two-element compound semiconductor. Accordingly, in the HEMT having the InGaAs channel layer and the ZnSe carrier supplying layer, the effect of preventing the real space carrier transfer to carrier supplying layer is small even when the energy difference between the conduction bands is large.